1. Field of the Invention
The present invention relates to a mask-frame assembly for use in a color-selecting-electrode system in color-television tubes, and to a method of manufacturing the same.
2. Description of the Prior Art
In a color-television tube of conventional design, the three electron beams must strike exactly their assigned red, green, and blue phosphor areas on screen. The necessary angle-dependent assignment of the phosphor areas to the electron beams is achieved by means of the so-called mask. For every trio of phosphor areas on the screen, there is one aperture in the mask, which is just behind the screen and parallel to it.
The requirements placed on the positional and dimensional accuracy of such a mask are very stringent, for the proper mask-screen relationship must be achieved over the entire area of the mask during production and then maintained over the whole life of the tube. In particular, there must be no change in position due to the temperature rise when the set is turned on. The resulting requirements can only be fulfilled by optimizing the design of the product and the manufacturing technology used, so that comparatively high costs are incurred.
In conventional methods, the mask and its frame are assembled after being blackened separately, so that dimensional changes due to the temperature of about 600.degree. C. during blackening can be largely compensated for during assembly. After being assembled by spot welding, the mask-frame assembly is stabilized at about 450.degree. C. to reduce internal stresses in the material which were caused by the welding process. The two thermal treatments of the mask and the frame in separate operations and facilities involve high handling costs and require two furnace installations. The separate stacking of masks and frames during the thermal treatments very easily results in distortion of the masks and frames under their dead weight, so that the masks and frames cannot be stacked to save factory space and reduce the amount of equipment required. When the mask and the frame are welded together and the mounting springs are welded on, splashes of welding flux are unavoidable because the surfaces had become poor conductors during the blackening process. Another disadvantage is that the hitherto used maximum temperature during blackening results in a high subsequent deformation ("collapsing") of the mask.